In a fuel injection system for an internal combustion engine, the fuel flow rate is determined basically in accordance with the rate of airflow inducted into the cylinders of the engine and the rotational speed (rpm) of the engine. The rate of airflow is controlled by a throttle valve disposed in the intake passage of the engine where the opening degree of the throttle valve is controlled by an accelerator pedal which is operatively connected thereto.
An airflow meter is usually employed for measuring the airflow rate and consists of a rotatable or pivotal flap disposed in the intake passageway where the flap is mechanically connected to a movable contact of a potentiometer. The flap is arranged to rotate against the biasing force of a spring under the influence of the pressure difference on the upstream side of the flap and the downstream side of same. The potentiometer is arranged to produce an output signal the voltage of which is indicative of the angular displacement of the flap and which is utilized for control of the pulse width of a pulse signal with which fuel injection valves are energized.
In such an air flow meter, a damper or a damping device is employed for reducing the flactuation of the movement of the flap. However, when the air flow rate increases or decreases abruptly, the movement of the flap is apt to be excessive to produce an overshoot phenomena and thus the potentiometer connected thereto produces an output signal indicative of an air flow rate which is higher or lower than the actual airflow rate. This erroneous signal causes the fuel injection system to supply more or less fuel respectively than necessary so that the air-fuel mixture becomes richer or leaner than a predetermined or desired value. Although a closed loop type air/fuel ratio control system is basically advantageous for avoiding undesirable variations of the air/fuel ratio, the closed loop system is easily influenced by such an erroneous signal since a time delay is inherent in the system. The undesirably enriched or impoverished air-fuel mixture causes an increase of the concentration of toxic components in the exhaust gases and also a decrease in the efficiency of a catalytic converter (if a three-way type), if disposed, in the exhaust system since such a catalytic converter exhibits its maximum efficiency when the air/fuel ratio of the air-fuel mixture is maintained within a narrow range (usually close to the stoichiometric value).
The above mentioned undesirable overshoot characteristics of the flap of the airflow meter can be reduced to a negligible extent by designing and adjusting the damper or the damping device carefully and precisely. However, such an airflow meter requires a complex construction and time consuming adjustment of the same. Therefore, the above mentioned provision of a complex damper for the reduction of the overshoot characteristics causes an increase in the cost of the air flow meter.
Although it is described hereinabove that overshoot phenomenon of the flap of the airflow meter occurs in case of abrupt increase and decrease of the airflow rate, the frequency of the abrupt decreases is considerably higher than that of the abrupt increases. When the air-flow rate decreases abruptly the fuel flow rate decreases. However, because of the overshoot characteristics of the flap of the airflow meter, the fuel flow rate falls below a required level so that a lean air/fuel mixture is supplied to the cylinders of the engine as mentioned hereinbefore. When the air/fuel ratio is lower than a predetermined value, the air/fuel mixture is apt to misfire or improperly ignite and thus the amount of unburnt gases emitted from the engine increases.
In a conventional fuel injection system, such as described in Japanese Patent pre-publications No. 52-18535 and No. 52-25932, the fuel flow rate is increased for a predetermined period of time when the throttle valve is fully or almost closed in the former or when the rate of decrease of the airflow is over a predetermined value in the latter. With this arrangement the lean mixture due to the overshoot characteristics of the flap of the airflow meter is compensated for. However, in the former case since the fuel flow rate is increased whenever the throttle valve is fully closed, the air/fuel ratio becomes higher than a desired level when the airflow rate decreases relatively gradually. In the latter case, since the fuel flow rate is increased whenever the rate of decrease of the airflow is over a predetermined value, the air/fuel ratio becomes higher than a desired level when the airflow rate increases after an abrupt decrease of the same. This means that in the conventional fuel injection system with a circuit which provides an increase of fuel flow rate for compensating for the air/fuel ratio, the fuel flow rate is increased not only in case it is necessary but also when it is unnecessary. Since the additional fuel is supplied to the engine undesirably, in case it is unnecessary, engine operation tends to be unstable and further fuel cost increases.